2. Melt butter in large saucepan over low heat. Add brown sugar; cook and stir until sugar is completely dissolved and smooth, about 4 minutes. Remove pan from heat and stir in cocoa until smooth. Add flour mixture to pan and stir until smooth. Stir in chocolate chips and vanilla. Beat in eggs. Spoon batter into prepared pan.

3. Bake 15 to 20 minutes or until toothpick inserted into center comes out almost clean. Cool completely on wire rack. Cut into bars.